This invention relates to an apparatus and a process for mass production of films by vacuum deposition.
There is the plasma CVD (chemical vapor deposition) process which appears before the footlights in recent years as a technique for producing a thin film. This process comprises evacuating a reaction chamber to a high vacuum, feeding a raw material gas into the chamber, then decomposing the gas by glow discharge, thereby forming a thin film on a substrate placed in the chamber. For instance, this process is applied to the production of amorphous silicon films. The amorphous silicon film prepared from a silane such as SiH.sub.4, Si.sub.2 H.sub.6 and the like by this process has relatively few localized levels in the forbidden band, its valence electron control is possible by doping it with a replacement type of impurity, and it can be produced to have excellent characteristics as an electrophotographic photosensitive member, so that much is expected therefrom.
The electrophotographic photosensitive member generally comprises a photoconductive layer and its substrate of so-called drum type. It is known that a batch type plasma CVD apparatus composed of a single vacuum chamber is employed for making such drum type photosensitive members.
This batch type plasma CVD apparatus has a structure, for example, as shown in FIG. 1. That is, the apparatus is constructed of a vacuum chamber 10 having a cover 1 and containing a cylindrical electrode 2 to which a high frequency voltage is to be applied and a drum type substrate 3 as an earthed counter electrode. The drum type substrate 3 on which a photoconductive film is to be formed can be heated with an internal heater 4; 5 is a rotary axis for revolving the substrate 3; 6 is a cylindrical shield for confining a plasma in a prescribed space inside the vacuum chamber; 9 is an evacuating system for the vacuum chamber 10; and 8 is a gas feeding system for introducing a feed gas into the vacuum chamber.
A plasma CVD film, e.g. an amorphous silicon film, is formed on the drum type substrate 3 by using such an apparatus in the following way:
The cover 1 is opened, a drum type substrate 3 is fixed around the rotary axis 5, the cover 1 is closed, and the vacuum chamber 10 is evacuated by the evacuating system 9 to a desired vacuum pressure. At the same time the drum type substrate 3 is heated to a temperature, for example, between about 200.degree. and about 300.degree. C. After the temperature has settled, a raw material gas is introduced from the gas-feeding system 8 into the vacuum chamber 10. The inside of the vacuum chamber is kept at a suitable pressure by controlling the feed gas flow and the sucked gas flow, and a high frequency voltage is applied to cylindrical electrode to cause a plasma in the space surrounded by the cylindrical shield 6. The feed gas, e.g. a silane, is decomposed and amorphous silicon deposits on the surface of the drum type substrate 3. After the deposit has grown to a desired thickness, the electric discharge and the introduction of gas are stopped and the heater 4 is switched off to leave the film-covered substrate 3 to cool.
Thereafter, air is introduced into the vacuum chamber 10, the cover 1 is opened, and the drum type substrate 3 on which a film is deposited is taken out.
In this film forming process, the initial evacuation and heating steps usually require 30 minutes-1 hour, the film forming step requires about 3 hours on the assumption that the film growing rate is about 10 .ANG./sec and the thickness of completed film is 10 .mu.m, and the cooling step requires for about 1-2 hours. Thus, the total time requirement at maximum is as much as about 6 hours, indicating poor productivity.
On the other hand, when an amorphous silicon film is formed by the plasma CVD process, a byproduct called polysilane is produced, which is in a fine powder form. In addition, when a thick film as much as about 10 .mu.m or more is formed, films form also on the cylindrical shield 6 and the cylindrical electrode 2. When the film forming process is repeated several times, these films will peel off and their fine fragments will scatter in the vacuum chamber 10. The polysilane powder and the fine fragments are blown up by the air flow at the introduction of air into the vacuum chamber 10 or at the evacuation of the chamber 10 after setting of the drum type substrate 3 thereinto, thus contaminating the surface of the substrate 3 and causing a deterioration of product quality.
Thus, according to the prior art, the disassembly and cleaning of the vacuum chamber is necessary after every film formation in order to minimize these effects of the powders.